The present invention relates to a process for compressing molding sand in a molding operation and, more particularly, a process for the effective compression of molding sand about complex, deep pocketed casting patterns.
European Patent Specification 0,022,808 discloses a process wherein molding sand which is introduced loosely into the molding space is precompressed by means of a pressure shock and thereafter post-compressed mechanically. The specific pressure required in the process and how they are applied by the pressure surge are not disclosed.
It is known from German Offenlegungsschrift 3,836,876 to compress with a single pressure shock which has a first low and then a second high pressure gradient (30 to 100 bar/sec. until approximately 1 to 3 bar is reached, and thereafter 100 to 600 bar/sec. until approximately 3 to 6 bar is reached). The pressure shock is applied so as to result final compression of the molding sand.
It is further known from U.S. Pat. No. 4,828,007 to compress with two pressure shocks, the first having a lower (up to a maximum of 40 bar/sec. ) pressure gradient than the second (up to 300 bar/sec.) at a maximum pressure of up to 20 bar, wherein a controlled pressure reduction taking place between the two pressure shocks in order to achieve a curve of dimensional stability decreasing towards the mold back.
DE 3,839,475 discloses a process for the compression of molding sands, containing binding clays, in a molding device, in which, before the pulse compression, some of the molding sand located in the molding frame, and particularly its increased packing density, is displaced towards the pulse generator counter to the effective direction of the pulse compression.
The displacement can be achieved in that, after the molding sand has been introduced into the molding chamber, gas, especially air, is introduced under pressure into the closed-off molding chamber and, as a result of the opening of at least one outlet orifice located in the wall of the molding chamber in the vicinity of the pulse generator, the gas contained within the molding sand introduced is expanded towards the outlet orifice and the molding sand is thus displaced towards the outlet orifice and therefore towards the pulse generator.
The foregoing process makes it possible to obtain in the molding chamber a distribution of the molding sand introduced which has a packing density which is low near the pattern and rises at an increasing distance from the pattern. Both the accelerating mass and the path available for the acceleration are therefore increased. At the same pulse intensity or the same acceleration, the speed of the mass striking the model device thereby increases, thus leading to a high impact force of the compression front during the pulse compression.
It is the principle object of the invention to provide a process for compressing molding sand wherein complicated casting patterns having deep pockets or the like can be better shaped and a more uniform compression can be achieved even in the more complicated and critical parts of the pattern, while, at the same time, ensuring favorable cycle times.